In the commercial aviation industry, it is well known that equipment that is critical for safety of flight must undergo extensive and expensive review by governmental aviation authorities so as to assure a very high level of reliability and availability. Once a piece of avionics equipment is fully approved, the desire to make small changes or upgrades, for example for aesthetic or economic improvements therein is often dominated and over shadowed by the high cost of recertification.
Cathode ray tubes (CRTs) are an example of equipment which is being replaced by liquid crystal displays (LCDs) in most newly designed commercial air transport aircraft. However, many newly manufactured previously designed aircraft are being made today with CRTs because of the high cost of recertification. One example would be the Engine Indicating and Crew Alerting System (EICAS) for Boeing 757 and 767 aircraft. The primary functions of the 757/767 EICAS are to monitor various engine and aircraft parameters and to provide indications of the values of those parameters to the crew, along with appropriate warnings and alerts triggered by the states of those parameters.
Now referring to FIG. 1, there is shown a portion of a prior art EICAS system. These functions are achieved by a system whose normal architecture includes the following Line Replaceable Units (LRU): two Engine Alert Processors (EAP), a single Display Select Panel (DSP), a Cancel/Recall Panel, a single Maintenance Panel, and two Display Units (DUs). This is the prior art configuration, the display units are CRTs.
The Engine Alert Processors (EAPs) receive and process various signals from the engines and the aircraft. These signals include both analog and digital data and control information. Each EAP in a given installation is identical, and is “strapped” via program pins for the particular engines on the aircraft. This functionality of the EAP is deeply ingrained in the currently certified EAPs.
The Display Select Panel (DSP) is the primary point of crew control for the EICAS. It receives analog discrete inputs from the Maintenance Panel and Cancel/Recall Panel and transmits this information to the EAPs to control the displayed format and parameters.
The DUs receive video signals from the EAPs and create the commanded displays. The DUs are identical, but independent in their operation. The DUs perform as a system with one DU presenting primary engine parameters continuously and caution and warning messages as required. The second DU continuously presents fuel flow, when Operational Program Configuration (OPC) is selected, and displays the remaining engine parameters when selected or automatically when the computer detects a parameter exceedance.
The left and right EAPs (in the center of the diagram) use inputs from the various interfacing systems to generate video signals that are routed to a switch relay card. Both EAPs produce video signals based on their inputs. The Display Select Panel (DSP) provides a means to determine which EAP will drive the DUs (only one EAP drives both DUs). The COMPUTER SELECT signal (shown in the diagram from the DSP to the switch relay card) determines which video signals will be used to drive the DUs.
A more detailed understanding of the prior art 757/767 EICAS EAP can be aided by now referring to FIGS. 2 and 3 which show an internal block diagram of the EAP and a description of the EAP's functions, together with a representation of the EAP data flow.
Engine Alert Processor (EAP)
Purpose:                Samples and conditions sensor input information.        Transmits the information to other systems.        Selects and constructs the pictures to appear on each DU.        Monitors the system's health and selects the system's response to combinations of faults.        
Structure:
The EAP is partitioned into three major subsystems. They are the System Processor (SP), the Input/Output Controller (IOC) and the Display Generator (DG). The three major subsystems operate independently and asynchronously except as described in Dynamic Operation. All communications between the three subsystems are accomplished by exchanging data in shared memories. The SP and the IOC share the Input/Output Buffer (IOB). It appears to each to be random access, read/write, volatile memory in their own address space. Several other hardware systems help these major subsystems function properly.
System Processor (SP)
The SP defines the functional and dynamic behavior of the system. It:                conditions sensor input data and formats it for display and transmission; selects the information to display on each DU.        alerts the crew to conditions requiring their attention.        tests itself, monitors the health of the rest of the system, and selects the system's responses to failures.        
I/O Controller (IOC)
The IOC:                transfers data between the SPs memory and the EAPs electrical interfaces except those associated with DUs.        conceals signal distribution and interface timing requirements from the SP.        converts between electrical and digital signal representations and vice versa, but does not condition or format the information.        is responsible for all functional requirements related to I/O signals.        
Display Generator (DG)
The DG:                translates the display information selected by the SP into deflection and video control signals while maintaining a sufficiently high refresh rate to avoid observable flickers.        
Program Memory (PM)                stores SP, IOC and DG instructions and constant data.        
Scratch Pad Memory (SPM)                stores variable data which does not need to be retained after a long power outage.        
Non-Volatile Memory (NVM)                A bank of memory used to store variable data which must be retained after an indefinitely long power outage.        
Video/Timing Generator (VTG)                provides a 20 Hz signal to the SP and DG.        provides all signals required by the display refresh circuitry.        
Power Supply (PS)                converts 400 Hz AC power for internal use.        provides signals required to manage power outages gracefully.        Two of these signals, PDNF and PLONGF, are routed to a register on the A11 card which the SP can read.        PLONGF, although logically is a signal from the power supply, it is an indicator of whether the external power went away for longer than 200 msec.        
Normal Operation:
The EAP data flow is represented in FIG. 3 and FIG. 4. The IOC and DG are essentially peripheral interfaces to the SP. The SP defines the functional and dynamic behavior of the system because it closes the data paths from system inputs to system and display outputs. The SP samples and conditions input data stored in the I/O Buffer (IOB) by the IOC. It updates output data in the IOB which the IOC transmits. The SP selects the displays to appear on each DU and updates Dynamic Information.
The IOC is nearly transparent to the SP. With the exception of DITS-33 Block Data, which does not concern much of the system, every input and output quantity is assigned a fixed location in the IOB. The SP treats each location as if it were directly attached to the transmitting/receiving device. In order to sample an input or update an output, the SP simply references the associated location in the IOB. Every quantity is independent of all others. The SP may reference them in any order at any rate regardless of the state of the IOC. If a quantity requires more than one word in the IOB, the IOC and the SP each read or update the entire quantity in one transaction so neither ever obtains a partially updated value.
The DG generates deflection and video signals to draw each of the displays selected by the SP. Based upon these selections, it processes the features to appear on each page.
Generally speaking, the SP, the IOC, and the DG operate independently. What little control the SP exercises over the IOC and the DG is accomplished through the Activity Interface words in shared memory.
The EAP as described above has been certified by the Federal Aviation Administration (FAA) and is in wide commercial use around the world. The EAP as described is commercially available for purchase directly from Rockwell Collins Inc., of Cedar Rapids, Iowa USA and is also available around the world from numerous spare parts and maintenance supplying companies.
While the EAP has enjoyed much commercial success in the past, in some applications the fact that the DUs are only CRTs may be considered today by some to be a shortcoming.
Consequently, there exists a need, in some applications, for improved EICAS systems for Boeing 757/767 aircraft.